After coronary occlusion myocardium originally supplied by the occluded artery ultimately separates into infarct and surviving muscle. To clarify circumstances leading to infarction, we compared the time course of collateral blood flow to muscle that later became infarct and collateral flow to muscle that survived despite inclusion within the distribution of the occluded artery. Permanent left anterior descending occlusion was produced in 14 dogs. Microspheres were given to measure blood flow prior to occlusion and 5 min, 20 min and 4 hr after occlusion. Dogs were sacrificed 3 days later and their hearts stained to identify the various tissue regions. Although blood flow was reduced to all regions supplied via collaterals, each subdivision of surviving muscle had higher flow than infarcting muscle as early as 5 min after coronary occlusion. Blood flow characteristics of each muscle type identify the fate of tissue very early after coronary occlusion. Blood flow increases as distance from infarcting muscle increases, with greater increase in tissue lying at the lateral borders of the region supplied by the occluded artery.